In ink-jet printing methods, droplets of ink are directly ejected onto a printing medium from very fine nozzles and allowed to adhere to the printing medium to form characters or images thereon. The ink-jet printing methods have become rapidly spread because of various advantages such as easiness of full coloration, low cost, capability of using a plain paper as the printing medium, non-contact with printed characters or images, etc.
In recent years, in order to impart good weathering resistance and good water resistance to printed materials, an ink containing a pigment as a colorant has been extensively used. In addition, from the standpoint of improving ejection stability and storage stability of ink, in order to stably compounding a pigment in a water-based ink, the technologies for enclosing the pigment in a polymer using a dispersant capable of modifying a surface of the pigment, etc., have been developed.
On the other hand, it has been required to print characters or images not only on a high-water absorbing printing medium such as those printing media conventionally called a plain paper and a copying paper, etc., but also on a printing medium for commercial printing purposes such as a low-water absorbing coated paper such as an offset coated paper or a non-water absorbing resin film such as a polyvinyl chloride resin film, a polypropylene resin film and a polyester resin film.
It is known that when characters or images are printed on the low-water absorbing or non-water absorbing printing medium by the ink-jet printing methods, there tend to occur problems such as prolonged drying time of the printed characters or images owing to slow absorption or no absorption of liquid components of the ink into the printing medium, and deterioration in rub fastness thereof in an initial stage of the printing.
Hitherto, when printing characters or images on the low-water absorbing or non-water absorbing printing medium, a solvent-based pigment ink using an organic solvent as a dispersing medium and a UV-curing ink have been mainly employed. This is because when using the aforementioned inks, there occurs such a phenomenon that the organic solvent is penetrated into the low-water absorbing or non-water absorbing printing medium and causes swelling of the surface of the printing medium to thereby fix the pigment in the printing medium, or a firm resin coating is formed on the printing medium after printing, so that it is possible to improve anti-bleeding properties and rub fastness of these inks.
However, these conventional inks tend to have such an inexpedience that a large amount of the organic solvent is diffused into air upon drying to cause adverse influence on environments, or a radical initiator or a monomer used in the UV-curing ink raises a risk concerning safety. For this reason, at the present time, development of water-based pigment inks having a less burden on working environments and natural environments has proceeded.
On the other hand, in the water-based pigment inks, it has been still required to improve their adhesion to a low-water absorbing printing medium or a non-water absorbing printing medium.
JP 2015-28114A (Patent Literature 1) discloses a water-based ink for ink-jet printing which is excellent in ejection properties, and capable of forming printed characters or images that are excellent in fixing properties onto a printing medium and obtaining printed materials having excellent gloss, and contains pigment particles and polyester resin particles, in which the pigment particles are constituted of a pigment and a polyester resin.
JP 2014-5448A (Patent Literature 2) discloses a two-component aqueous ink set for ink-jet printing which is free of disturbance of printed characters or images such as bleeding and intercolor bleeding, and excellent in rub fastness immediately after printing, and includes a first ink that contains a coloring material, a hydroxy group- and/or carboxy group-containing resin, and a water-soluble solvent and/or water, and a second ink that contains a vinylamine-based polymer, and a water-soluble solvent and/or water.